Process for synthesis of nucleoside analogues

ABSTRACT

The present invention is concerned with a process for the preparation of antiviral 1,3-oxathiolane nucleosides comprising an intramolecular glycosylation reaction to produce exclusively the β-diastereomer, and intermediates useful in the process.

This application is filed pursuant to 35 U.S.C. § 371 as a U.S. NationalPhase Application of International Application No. PCT/EP96/01353 filedMar. 26, 1996 which claims priority from GB9506644.5 filed Mar. 31,1995.

The present invention is concerned with a process for the preparation ofanti-viral 1,3-oxathiolane nucleosides, which employs an intramolecularglycosylation to produce exclusively the β-diastereomer. The inventionalso relates to novel intermediates obtained by the process.

1,3-Oxathiolane nucleosides possess two chiral centres (at the C1'- andC4'-positions according to the furanose numbering system) and typicallyexist as diastereomeric pairs of the α- and β-forms, each formcomprising two enantiomers. The α- and β-diastereoisomers tend to havedifferent anti-viral activities, the β-form typically being the morepotent. Similarly, the enantiomeric pairs of each diastereomer tend tohave different properties.

β-Diastereomers have traditionally been obtained by preparation of thediastereomeric mixture followed by laborious separation of the β-form byphysical means such as differential solubility or chromatography. Itfollows that the overall yield of β-isomer is typically less than 50%.

International Patent Application No. WO91/11186 (U.S. Pat. No.5,204,466) describes a process whereby 1,3-oxathiolane nucleosides maybe obtained with high β-diastereoselectivity by condensing acarbohydrate or carbohydrate-like moiety with a heterocyclic base in thepresence of a specific Lewis acid, typically stannic chloride. Theprocess is further exemplified in International Patent Application No.WO92/14743.

Further diastereoselective processes for the preparation of nucleosideanalogues involving condensation of a carbohydrate or like moiety with apurine or pyrimidine base are described in WO92/20669 (U.S. Pat. No.5,756,706) and WO95/29174.

We have now developed an efficient new process which providesexclusively the β-diastereomer of a 1,3-oxathiolane pyrimidinenucleoside with no α-contamination. The critical steps involved in thesynthesis are cyclisation of an appropriate heterocyclic acetaldehydewith 1,4-dithiane-2,5-diol to give a "5'-tethered" 1,3-oxathiolanenucleoside analogue which then undergoes an intramolecular glycosylationon the same face of the carbohydrate ring to give exclusively the(1'-tethered) β-diastereomer. The intramolecular glycosylation of5'-tethered furanose nucleosides is known from, inter alia, JapanesePatent No. 06263792-A, but the prior art comprises no reports ofapplying such methodology to the preparation of anti-viral1,3-oxathiolane nucleosides. The resulting β-diastereomer may behydrolysed to the corresponding cytidine analogue or may be resolved byany suitable technique known to a skilled person, for example, byesterification followed by selective enzymatic hydrolysis, removal ofthe `unwanted` enantiomer and hydrolysis of the ester of desiredenantiomeric configuration. Alternatively, it may be possible, forexample, by use of a chiral auxiliary, to obtain intermediatessubstantially enantiomerically pure which intermediates can be carriedforward to yield the desired enantiomerically pure product.

According to one aspect of the present invention, there is provided aprocess for the preparation of compounds of formula (I) ##STR1## whereinR is hydrogen, C₁₋₆ alkyl, or halogen and Y is hydroxy, amino, C₁₋₆alkoxy or OR¹, where R¹ is a chiral auxiliary, which process comprisestreating a compound of formula (II) ##STR2## wherein R and Y are ashereinbefore defined and R² represents hydrogen, C₁₋₆ acyl, C₁₋₆ alkylor halogen with a suitable Lewis acid or a reagent apt to convert thegroup OR² to a leaving group.

Suitable Lewis acids include, for example, stannic chloride ortrimethylsilyl triflate. Reaction with a Lewis acid is suitablyconducted at reduced temperature (e.g. 0° C. to -20° C.) in a polaraprotic solvent followed by treatment with base.

Where R² is H, the group OR² may conveniently be converted to a leavinggroup by reaction with a halogenating agent such as a thionyl halide oran oxalyl halide, or a tosyl or mesyl halide. Other methods forconverting OR² to a leaving group (i.e. a group which can be readilydisplaced by the ring nitrogen atom) will be apparent to those skilledin the art.

It is to be understood that where the variable R occurs more than oncein a general formula, it may represent the same group at each position,or different groups.

As used herein halogen means bromine, chlorine, fluorine or iodine,especially chlorine or fluorine, more especially fluorine.

The term "chiral auxiliary" describes an asymmetric molecule that isused to effect the chemical resolution of a racemic mixture. Such chiralauxilliaries may possess one chiral centre such as α-methylbenzylamineor several chiral centres such as menthol. The purpose of the chiralauxiliary, once built into the starting material, is to allow simpleseparation of the resulting diastereomeric mixture. See, for example, JJacques et al., Enantiomers, Racemates and Resolutions, pp. 251-369,John Wiley & Sons, New York (1981).

Where R¹ represents a chiral auxiliary it will preferably be selectedfrom (d)-menthyl, (I)-menthyl, (d)-8-phenylmenthyl, (I)-8-phenylmenthyl,(+)-norephedrine and (-)-norephedrine. More preferably R¹ is(I)-menthyl, or (d)-menthyl, most preferably (I)-menthyl.

According to a further aspect, the present invention provides a processfor the preparation of a compound of formula (Ia) ##STR3## wherein R andY are as previously defined, which process comprises treating a compoundof formula (IIa) ##STR4## wherein R, Y and R² are as previously definedwith a suitable Lewis acid or a reagent apt to convert the group OR² toa leaving group.

According to another aspect of the invention, there is provided aprocess for the preparation of compounds of formula (II) which comprisesreacting a compound of formula (III) ##STR5## wherein R and Y are ashereinbefore defined, with 1,4-dithiane-2,5-diol at elevated temperature(e.g. 100° C.) in a non-polar aprotic solvent to give a compound offormula (II) wherein R² is H. Compounds of formula (II) wherein R² isother than H may be prepared from the corresponding hydroxy compound byderivatisation using any standard procedure, for example, treatment withalkanoyl halide/base or carboxylic anhydride/base.

Reaction of a compound of formula (III) with 1,4-dithiane-2,5-diolresults in a mixture of isomers of the compounds of formula (II) whereinR² is H. Where Y is OR¹, the compounds of formula (IIa) may beselectively crystalized from the diastereomeric mixture. In a further oralternative aspect, the present invention accordingly provides a methodfor obtaining the compound of formula (IIa) wherein R is H and Y is OR¹from a mixture of isomers by treatment of the mixture of isomers, atleast partially in solution, with an agent capable of effectinginterconversion of the isomers without complete suppression of thecrystallisation of the desired single enantiomer (IIa) wherein R is Hand Y is OR¹. Other compounds of formula (IIa) may be prepared fromcompounds of formula (IIa) wherein R is H and Y is OR¹ by conventionalmethods.

Agents capable of effecting interconversion of the isomers withoutcomplete suppression of the crystallisation of the trans isomersinclude, for example, alcohols, such as, for example, methanol, ethanol,n-propanol, i-propanol, n-butanol, i-butanol, t-butanol, and organicbases, in particular tertiary amines, for example, pyridine andtriethylamine and Hunig's base. A preferred agent is triethylamine.

The interconversion of isomers may be effected in any suitable solventor mixture of solvents which does not otherwise react with the alcoholsof formula (II), under conditions of concentration and temperature whichpermit crystallisation of the desired isomer or isomers and which do notcause significant degradation of the desired isomer or isomers. Suitablesolvents may include for example, aliphatic or aromatic hydrocarbons,ethers, esters and chlorinated hydrocarbons. The interconversion willpreferably be effected at a temperature of about -20° to 120° C., morepreferably in the range of about -10° to 80° C., such as about 0° to 50°C.

It will be appreciated by those skilled in the art that selection ofsolvent, temperature, interconversion agent and, particularly, thequantity of the interconversion agent is best conducted as an integratedexercise dependent on the nature of the groups R, R¹ and R² present inthe isomers. However, when an organic base is used as theinterconversion agent, the preferred quantity is generally less than twomole-equivalents based on the total of all isomers of (II) present.

The interconversion of isomers may be conducted separately from thepreparation of the isomeric mixture; however, it is convenientlyconducted concomitantly with that preparation.

The interconversion procedure may also be used to increase the isomericpurity of isolated (IIa).

By means of the interconversion process, the isolated yield of thedesired isomer (IIa) may be enhanced lo greater than 50% of theory(based on formation of all stereoisomers), typically to between about60% and about 90% of theory; but it is not ruled out that yieldsapproaching 100% of theory may be obtained.

Compounds of formula (III) may be prepared by reacting a compound offormula (IV) ##STR6## wherein R (which may be the same or different) andY are as hereinbefore defined, with aqueous trifluoroacetic acid (90%)at elevated temperature.

Compounds of formula (IV) may be prepared by reacting a compound offormula (V) ##STR7## wherein R and Y are as hereinbefore defined and Zis a suitable leaving group, for example, chlorine, with a compound offormula (VI) ##STR8## wherein R (which may be the same or different) areas hereinbefore defined, at reduced temperature in a polar aproticsolvent in the presence of base.

Compounds of formula (V) may be prepared by reacting a compound offormula (VII) ##STR9## wherein R and Z (which may be the same ordifferent) are as hereinbefore defined, with a suitable nucleophile, forexample, in the case where Y in the compound of formula (V) is to beethoxy, EtO⁻ (NaOEt/EtOH).

Compounds of formulae (VI) and (VII) may be obtained commercially orprepared from commercially available starting materials by methods knownto a skilled person, for example, in the case where R in the compound offormula (VII) is to be fluorine and Z chlorine, by treating5-fluorouracil with phosphorus oxychloride at elevated temperature inthe presence of base.

As indicated, compounds of formula (I) wherein Y at the C4-position isC₁₋₆ alkoxy or OR¹ may be converted to a cytidine analogue (Y=NH₂) byheating with ammoniacal methanol or, where racemic, may be resolved byany suitable technique known to a skilled person, for example, by one ofthe enzyme procedures described in International Patent No. WO92/14743.

According to such a procedure, the racemic β-diastereomer (I) isesterified at the C5'-position using, for example, butyric anhydride,and the racemic ester (VIII) is treated with a suitable enzyme,typically pig liver esterase, to preferentially hydrolyse the `unwanted`enantiomer back to the 5'-OH compound (IX) which is water-soluble andcan be separated from the desired (unhydrolysed) enantiomer (X). Thelatter is converted to the 4-NH₂, 5'-OH compound of desired enantiomericconfiguration by heating with ammoniacal methanol. ##STR10##

The process of the invention finds particular application in thepreparation of(2R,5S)-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine,(2R,5S)-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine,(±)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-cytosine and(±)-cis-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine.

According to a further aspect of the invention, there are provided novelcompounds of formulae (IV), (III), (II) and (I) (which latter includesthe racemate, the (2S,5R)-enantiomer (IX), the esterified racemate(VIII) and the esterified (2R,5S)-enantiomer (X)). Specific intermediatecompounds arising from the preparation of(2R,5S)-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine,(2R,5S)-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine,(±)-cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-cytosine and(±)-cis-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine include:

2-(2,2-Dimethoxyethoxy)-4-ethoxy-5-fluoropyrimidine

2-(2,2-Dimethoxyethoxy)-4-ethoxypyrimidine

2-[(4-Ethoxy-5-fluoro-2-pyrimidinyl)oxy]acetaldehyde

2-[(4-Ethoxy-2-pyrimidinyl)oxy]acetaldehyde

2-{[(4-Ethoxy-5-fluoro-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ol

2-{[(4-Ethoxy-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ol

2-{[(4-Ethoxy-5-fluoro-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ylacetate

2-{[(4-Ethoxy-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-yl acetate

(2S*,5R*)-4-Ethoxy-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

(2S*,5R*)-4-Ethoxy-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

(2S*,5R*)-4-Ethoxy-5-fluoro-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

(2S*,5R*)-4-Ethoxy-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

(2S,5R)-4-Ethoxy-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

(2S,5R)-4-Ethoxy-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

(2R,5S)-4-Ethoxy-5-fluoro-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

(2R,5S)-4-Ethoxy-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

The following examples of the process of the invention are forillustration only and are not intended to limit the scope of theinvention in any way. In all cases, ¹ H NMR and C,H,N elemental analysiswere consistent with the proposed structure.

EXAMPLE 1 Preparation of(2R,5S)-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine

(a) 2,4-Dichloro-5-fluoropyrimidine

To a suspension of 5-fluorouracil (Aldrich, 8.00 g, 61.5 mmol) inphosphorus oxychloride (25.0 mL, 41.12 g, 268 mmol) was addedN,N-diethylaniline (12.6 mL, 11.81 g, 80 mmol) and the mixture washeated at 100° C. for 1.5 hours. Solvent was evaporated in vacuo and theresidue poured into cold H₂ O/Et₂ O (400 mL, 1:1). The aqueous phase wasextracted with Et₂ O and the combined organic phase was dried (Na₂ SO₄)and evaporated (water aspirator pump, 35° C.) to give the desiredproduct (10.2 g, 99%) as a yellowish solid: mp 34-36° C. (lit. 35-36°C.).

(b) 2-Chloro-4-ethoxy-5-fluoropyrimidine

To a solution of the product from step (a) (10.0 g, 59.9 mmol) in abs.EtOH (40 mL) at 0° C. under nitrogen atmosphere was added 1M NaOEt/EtOH(61 mL, 61 mmol) and the mixture was stirred for 1 hour. Solvent wasevaporated in vacuo and the residue partitioned between H₂ O and Et₂ O.The aqueous phase was extracted with Et₂ O and the combined organicphase was washed with brine, dried (Na₂ SO₄) and evaporated (wateraspirator pump, 35° C.) to provide the desired product (8.74 g, 83%) asa yellowish solid: mp 30-32° C. (lit. 31-32° C.); ¹ H NMR (CDCl₃): δ1.46(t, J=7.0 Hz, 3H), 4.53 (quartet, J=7 Hz, 2H), 8.17 (d, J=2.1 Hz, 1H);MS m/z 179 (M+3, 17%), 177 (M+1, 50%), 149 (100%). Anal. Calcd. for C₆H₆ ClFN₂ O: C, 40.81; H, 3.42; N, 15.86. Found, C, 40.90; H, 3.45; N,15.81.

(c) 2-(2,2-Dimethoxyethoxy)-4-ethoxy-5-fluoropyrimidine

To a suspension of 60% NaH/mineral oil (2.88 g, 72.2 mmol) in anhydrousDMF (70 mL) at 0° C. under nitrogen atmosphere was slowly addedglycolaldehyde dimethyl acetal (Lancaster, 6.13 g, 57.7 mmol). Themixture was stirred at ambient temperature for 1 hour and thentransferred to a solution of the product from step (b) (8.5 g, 48.1mmol) in anhydrous DMF (70 mL) at -55° C. over 15 minutes. The mixturewas allowed to warm to -20° C. over 2 hours and then neutralized withAcOH. Solvent was evaporated in vacuo and the residue partitionedbetween H₂ O and CH₂ Cl₂. The aqueous phase was extracted with CH₂ Cl₂and the combined organic phase was dried (Na₂ SO₄) and evaporated. Theresidue was flash chromatographed (EtOAc/hexanes, 1:5) to give thedesired product (9.75 g, 82%) as an oil: ¹ H NMR (CDCl₃): δ1.42 (t,J=7.0 Hz, 3H), 3.43 (s, 6H), 4.32 (d, J=5.2 Hz, 2H), 4.50 (quartet,J=7.0 Hz, 2H), 4.75 (t, J=5.2 Hz, 1H), 8.03 (d, J=2.4 Hz, 1H); MS m/z215 (M-OCH₃, 100%). Anal. Calcd. for C₁₀ H₁₅ FN₂ O₄ : C, 48.78; H, 6.14;N, 11.38. Found: C, 48.84; H, 6.06; N, 11.36.

(d) 2-[(4-Ethoxy-5-fluoro-2-pyrimidinyl)oxy]acetaldehyde

A mixture of the product from step (c) (6.0 g, 24.4 mmol) and 90% TFA/H₂O (50 ml) was heated at 50° C. for 2.5 hours. Solvent was evaporated invacuo and the residue partitioned between CHCl₃ and saturated NaHCO₃ /H₂O. The aqueous phase was extracted with CHCl₃ (×2) and the combinedextracts dried (Na₂ SO₄) and evaporated to give the desired product(4.82 g, 99%) as a colourless oil which was used in the next stepwithout further purification. Flash chromatography (EtOAc/hexanes, 1:2)gave analytically pure material as a colourless oil: ¹ H NMR (CDCl₃):δ1.43 (t, J=7.0 Hz, 3H), 4.40 (quartet, J=7.0 Hz, 2H), 4.81 (s, 2H),8.03 (d, J=1.8 Hz, 1H), 9.74 (s, 1H); MS m/z 201 (M+1, 100%). Anal.Calcd. for C₈ H₉ FN₂ O₃.0.25 H₂ O: C, 46.95; H, 4.68; N, 13.69. Found:C, 46.81; H, 4.61; N, 13.64.

(e) 2-{[(4-Ethoxy-5-fluoro-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ol

A mixture of the product from step (d) (4.6 g, 23.0 mmol) and1,4-dithiane-2,5-diol (Aldrich, 1.92 g, 12.65 mmol) in anhydrous toluene(90 mL) was heated at 100° C. for 2 hours. The mixture was filtered andthe filtrate was concentrated and dried in vacuo to give the desiredproduct (6.27 g, 99%) as a waxy pale yellow solid which was used in thenext step without further purification (˜1:1 diastereomeric ratio by ¹ HNMR spectroscopy). Flash chromatography (EtOAc/hexanes, 1:2) affordedanalytically pure material as a white solid: mp 85-87° C.; ¹ H NMR(CDCl₃): δ1.41 (t, J=7.0 Hz, 3H), 2.42 (br s, 1H), 3.10 (d, J=11.0 Hz,1H), 3.20 (dd, J=11.0, 3.5 Hz, 1H), 4.40 (dd, J=12.0, 3.5 Hz, 1H), 4.43(quartet, J=7.0 Hz, 2H), 4.77 (dd, J=12.0, 7.0 Hz, 1H), 5.70 (dd, J=7.0.3.5 Hz, 2H), 5.92 (d, J=3.5 Hz, 1H), 8.04 (d, J=2.5 Hz, 1H); a similarset of signals appeared for the other diastereomer; MS m/z 201 (M-C₂ H₃OS, 100%). Anal. Calcd. for C₁₀ H₁₃ FN₂ O₄ S: C, 43.47; H, 4.74; N,10.14; S, 11.61. Found: C, 43.56; H, 4.78; N, 10.04; 11.66.

(f) 2-{[(4-Ethoxy-5-fluoro-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ylacetate

To a solution of the product from step (e) (1.0 g, 3.62 mmol) andpyridine (0.8 mL, 0.78 g, 9.88 mmol) in CH₂ Cl₂ (12 mL) at 0° C. wasadded AcCl (0.35 mL, 0.37 g, 4.7 mmol). After 1 hour at ambienttemperature, saturated NaHCO₃ /H₂ O was added and the aqueous phase wasextracted with CHCl₃. The combined organic phase was washed with brine,dried (Na₂ SO₄), evaporated and dried in vacuo to give the desiredproduct (1.13 g, 99%) as a yellow oil which was used in the next stepwithout further purification (˜2:1 diastereomeric ratio by ¹ H NMRspectroscopy). Flash chromatography (acetone/CH₂ Cl₂, 1:24) gaveanalytically pure material as a colourless oil: ¹ H NMR (CDCl₃): δ1.42(t, J=7.0 Hz, 3H), 2.07 (s, 3H), 3.15 (d, J=11.5 Hz, 1H), 3.38 (dd,J=11.5, 4.0 Hz, 1H), 4.40-4.60 (m, 4H), 5.73 (m, 1H), 6.70 (d, J=4.0 Hz,1H), 8.03 (d, J=2.5 Hz, 1H); a similar set of signals appeared for theminor diastereomer; MS m/z 259 (M-OAc, 9%), 159 (100%). Anal. Calcd. forC₁₂ H₁₅ FN₂ O₅ S: C, 45.28; H, 4.75; N, 8.80; S, 10.07. Found: C, 45.35;H. 4.76; N, 8.83; S, 10.11.

(g)(2S*,5R*)-4-Ethoxy-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

To a mixture of the product from step (f) (0.21 g, 0.66 mmol) and 4 Åmolecular sieves (0.3 g) in anhydrous CH₃ CN (20 mL) at -20° C. undernitrogen atmosphere was slowly added trimethylsilyl triflate (Aldrich,0.14 mL, 0.16 g, 0.73 mmol). After stirring the mixture for 2 hours at-20° C., 1M NaOH/H₂ O (2.0 mL, 2.0 mmol) was added. After 2 hours at 0°C., the mixture was neutralized with AcOH. Solvent was evaporated invacuo and the residue flash chromatographed (EtOAc/hexanes, 9:1) to givethe desired product (0.11 g, 60%) as a white solid: mp 162-164° C.; ¹ HNMR (DMSO-d₆): δ1.39 (t, J=7.0 Hz, 3H), 3.29 (dd, J=12.0, 2.7 Hz, 1H),3.60 (dd, J=12.0, 5.4 Hz, 1H), 3.82 (ddd, J=12.5, 5.4, 3.5 Hz, 1H), 3.95(ddd, J=12.5, 5.4, 3.5 Hz, 1H), 4.45 (quartet, J=7.0 Hz, 2H), 5.31 (t,J=3.5 Hz, 1H), 5.63 (t, J=5.4 Hz, 1H), 6.20 (m, 1H), 8.74 (d, J=6.7 Hz,1H); MS m/z 277 (M+1, 4%), 159 (100%). Anal. Calcd. for C₁₀ H₁₃ FN₂ O₄S: C, 43.47; H, 4.74; N, 10.14; S, 11.61. Found: C, 43.54; H, 4.76; N,10.18; S, 11.52.

(h)(2S,5R*)-4-Ethoxy-5-fluoro-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]-pyrimidin-2-one

To a solution of the product from step (g) (90 mg) in pyridine (0.2 mL)was added butyric anhydride (1.0 mL) and the resulting mixture wasstirred at ambient temperature for 18 hours. Ice-water was added and theaqueous solution was adjusted to pH 2 with 1N HCl/H₂ O and extractedwith CHCl₃ (×3). The combined organic phase was washed with saturatedNaHCO₃ /H₂ O and brine, dried (Na₂ SO₄) and concentrated in vacuo. Theresulting oil was dried in vacuo at 50° C. for 18 hours under a streamof nitrogen to obtain the desired product (100 mg) as a colourlesssolid: ¹ H NMR (CDCl₃): δ0.99 (t, 3H), 1.42 (t, 3H), 1.70 (sextuplet,2H), 2.42 (t, 2H), 3.23 (d, 1H), 3.60 (dd, 1H), 4.45 (dd, 1H), 4.50(quartet, 2H), 4.65 (dd, 1H), 5.40 (m, 1H), 6.30 (m, 1H), 8.15 (d, 1H);MS m/z 347 (M+1, 25%), 159 (100%).

(i)(2R,5S)-4-Ethoxy-5-fluoro-1-[2-(butanoyloxymethyl)-1-3-oxathiolan-5-yl]pyrimidin-2-one

To a solution of the product from step (h) (10 mg) in 20% CH₃ CN/buffer(3.0 mL, 0.05 M, pH 8.0, phosphate) was added PLE (pig liver esterase,1.5 μL, Sigma) and the mixture was stirred at ambient temperature for 24hours. The aqueous solution was extracted with hexane (×2) and thecombined extracts were dried (Na₂ SO₄) and concentrated in vacuo. HPLCanalysis (Chiral Pack AS; EtOH; 1.5 ml/min) of the organic extractsindicated the presence of a single enantiomeric butyrate ester (4 mg).The enantiomeric alcohol was detected in the aqueous phase. Ester: ¹ HNMR (CDCl₃): δ0.97 (t, J=7.4 Hz, 3H), 1.42 (t, J=7.0 Hz, 3H), 1.67(sextuplet, J=7.4 Hz, 2H), 2.40 (t, J=7.4 Hz, 2H), 3.23 (d, J=12.8 Hz,1H), 3.60 (dd, J=12.8, 5.3 Hz, 1H), 4.46 (dd, J=12.6, 2.5 Hz, 1H), 4.52(quartet, J=7.0 Hz, 2H), 4.65 (dd, J=12.6, 4.0 Hz, 1H), 5.37 (m, 1H),6.29 (m, 1H), 8.12 (d,J=6Hz, 1H).

(j) (2R,5S)-5-Fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine

A solution of the ester from step (i) (4 mg) in NH₃ /MeOH (2 mL) wasplaced in a steel bomb with a teflon liner, sealed and heated at 70° C.for 18 hours. Solvent was evaporated in vacuo to provide the desiredproduct (2 mg) with HPLC, ¹ H NMR and MS properties identical to thoseof an authentic sample.

EXAMPLE 2 Preparation of(2R,5S)-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine

(a) 2-Chloro-4-ethoxypyrimidine

To a solution of 2,4-dichloropyrimidine (Aldrich, 10.0 g, 67.12 mmol) inabs. EtOH (120 mL) at -3° C. under nitrogen atmosphere was slowly added(over 2 hours) 1M NaOEt/EtOH (68 mL, 68 mmol) and the resulting mixturestirred for 1 hour. Solvent was evaporated in vacuo and the residuepartitioned between H₂ O and Et₂ O. The aqueous phase was extracted withEt₂ O and the combined organic phase was washed with brine, dried (Na₂SO₄) and evaporated (water aspirator pump, 35° C.). The resultingresidue was filtered and washed with petroleum ether to provide thedesired product (8.05 g, 75%) as a yellowish solid: mp 30-31° C. (lit.35° C.); ¹ H NMR (CDCl₃): δ1.40 (t, J=7.2 Hz, 3H), 4.44 (quartet, J=7.2Hz, 2H), 6.62 (d, J=5.7 Hz, 1H), 8.27 (d, J=5.7 Hz, 1H); MS m/z 161(M+3, 34%), 159 (M+1, 100%). Anal. Calcd. for C₆ H₇ ClN₂ O: C, 45.44; H,4.45; N, 17.66; Cl, 22.36. Found: C, 45.32; H, 4.41; N, 17.60; Cl,22.43.

(b) 2-(2,2-Dimethoxyethoxy)-4-ethoxypyrimidine

To a suspension of 60% NaH/mineral oil (2.55 g, 63.96 mmol) in anhydrousDMF (70 mL) at 0° C. under nitrogen atmosphere was slowly addedglycolaldehyde dimethyl acetal (Aldrich, 5.65 g, 53.3 mmol). The mixturewas stirred at ambient temperature for 1 hour and then transferred to asolution of the product from step (a) (8.05 g, 50.76 mmol) in anhydrousDMF (70 mL) at -55° C. over 15 minutes. The mixture was allowed to warmto -20° C. over 2 hours and then neutralized with AcOH. Solvent wasevaporated in vacuo and the residue partitioned between H₂ O and CH₂Cl₂. The aqueous phase was extracted with CH₂ Cl₂ and the combinedorganic phase was dried (Na₂ SO₄) and evaporated in vacuo. The residuewas flash chromatographed (EtOAc/hexanes, 1:4) to give the desiredproduct (7.92 g, 69%) as a colourless oil: ¹ H NMR (CDCl₃): δ1.37 (t,J=7.0 Hz, 3H), 3.44 (s, 6H), 4.36-4.43 (m, 4H), 4.78 (t, J=5.0 Hz, 1H),6.34 (d, J=6.0 Hz, 1H), 8.15 (d, J=6.0 Hz, 1H); MS m/z 229 (M=1, 13%),197 (100%). Anal Calcd. for C₁₀ H₁₆ N₂ O₄ : C, 52.62; H, 7.07; N, 12.27.Found: C, 52.45; H, 7.01; N, 12.26.

(c) 2-[(4-Ethoxy-2-pyrimidinyl)oxy]acetaldehyde

A mixture of the product from step (b) (6.0 g, 24.4 mmol) and 90% TFA/H₂O (45 ml) was heated at 50° C. for 2 hours. Solvent was evaporated invacuo and the residue partitioned between CHCl₃ and saturated NaHCO₃ /H₂O. The aqueous phase was extracted with CHCl₃ (×2) and the combinedextracts were dried (Na₂ SO₄), and evaporated in vacuo to give thedesired product (4.48 g, 94%) as a colourless oil: ¹ H NMR (CDCl₃):δ1.38 (t, J=7.0 Hz, 3H), 4.37 (quartet, J=7.0 Hz, 2H), 4.80 (s, 2H),6.40 (d, J=6.0 Hz, 1H), 8.15 (d, J=6.0 Hz, 1H), 9.74 (s, 1H); MS m/z 183(M+1, 100%). Anal. Calcd. for C₈ H₁₀ N₂ O₃.0.25 H₂ O: C, 51.47; H, 5.67;N, 15.01. Found: C, 51.38; H, 5.69; N, 14.76.

(d) 2-{[(4-Ethoxy-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ol

A mixture of the product from step (c) (4.0 g, 22.0 mmol) and1,4-dithiane-2,5-diol (Aldrich, 1.67 g, 11.0 mmol) in anhydrous toluene(80 mL) was heated at 100° C. for 2 hours. The mixture was filtered andthe filtrate concentrated and dried in vacuo to give the desired product(6.27 g, 99%) as a waxy pale yellow oil which was used in the next stepwithout further purification (˜1:1 diastereomeric ratio by ¹ H NMRspectroscopy). Flash chromatography (EtOAc/hexanes, 2:3) gaveanalytically pure material as a colourless oil: ¹ H NMR (CDCl₃): δ1.37(t, J=7.0 Hz, 3H), 3.07 (d, J=11.0 Hz, 1H), 3.18 (d, J=2.3 Hz, 1H), 3.26(dm, J=11.0 Hz, 1H), 4.38-4.58 (m, 3H), 4.85 (dd, J=12.0, 6.0 Hz, 1H),5.72 (dd, J=6.0, 4.5 Hz, 1H), 5.92 (m, 1H), 6.39 (d, J=6.0 Hz, 1H), 8.15(d, J=6.0 Hz, 1H); a similar set of signals appeared for the otherdiastereomer; MS m/z 197 (M-C₂ H₅ O, 41%), 133 (100%). Anal. Calcd. forC₁₀ H₁₄ N₂ O₄ S: C, 46.50; H, 5.46; N, 10.85; S, 12.41. Found: C, 46.40;H, 5.44; N, 10.79; S, 12.49.

(e) 2-{[(4-Ethoxy-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-yl acetate

A mixture of the product from step (d) (1.0 g, 3.9 mmol), pyridine (0.7mL, 0.68 g, 8.65 mmol) and Ac₂ O (2.0 mL, 2.26 g, 21.2 mmol) was stirredat ambient temperature for 1.5 hours. Ice-water was added and theresulting mixture stirred for 15 minutes. The mixture was extracted withEtOAc and the combined extracts washed with saturated NaHCO₃ /H₂ O,dried (Na₂ SO₄), evaporated and dried in vacuo to give the desiredproduct (1.15 g, 99%) as an orange oil which was used in the next stepwithout further purification (˜2:1 diastereomeric ratio by ¹ H NMRspectroscopy). ¹ H NMR (CDCl₃): δ1.40 (t, 3H), 2.05 (s, 3H), 3.08 (d,1H), 3.27 (dd, 1H), 4.40-4.70 (m, 4H), 5.79 (m, 1H), 6.38 (d, 1H), 6.75(d, 1H), 8.18 (d, 1H); a similar set of signals appeared for the minordiastereomer; MS m/z 241 (M-OAc, 4%), 141 (100%). Anal. Calcd. for C₁₂H₁₆ N₂ O₅ S: C, 47.99; H, 5.37; N, 9.33; S, 10.68. Found: C, 47.88; H,5.43; N, 9.22; S, 10.60.

(f)(2S*,5R-)-4-Ethoxy-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

To a solution of the product from step (e) (0.20 g, 0.66 mmol) inanhydrous CH₃ CN (12 mL) at 0° C. under nitrogen atmosphere was slowlyadded stannic chloride (Aldrich, 0.12 mL, 0.27 g, 1.05 mmol). Afterstirring for 2 hours at 0° C., 1M NaOH/H₂ O (5.5 ml, 5.5 mmol) wasadded. After 1 hour at 0° C., the mixture was neutralized with AcOH.Solvent was evaporated in vacuo and the residue partitioned betweenCHCl₃ and water. The aqueous phase was extracted with CHCl₃ (×2) and thecombined extracts were dried (Na₂ SO₄) and concentrated in vacuo. Theresidue was flash chromatographed (EtOAc/hexanes, 2:1, then EtOAc) togive the desired product (0.10 g, 60%) as a white solid: mp 117-118° C.;¹ H NMR (DMSO-d₆): δ1.26 (t, J=7.0 Hz, 3H), 3.15 (dd, J=12.0, 3.5 Hz,1H), 3.51 (dd, J=12.0, 5.5 Hz, 1H), 3.71-3.84 (m, 2H), 4.26 (quartet,J=7.0 Hz, 2H), 5.22 (t, J=4.0 Hz, 1H), 5.40 (t, J=6.0 Hz, 1H), 6.0 (d,J=7.4 Hz, 1H), 6.18 (dd, J=5.5, 3.5 Hz, 1H), 8.25 (d, J=7.4 Hz, 1H); MSm/z 259 (M+1, 4%), 141 (100%). Anal. Calcd. for C₁₀ H₁₄ N₂ O₄ S: C,46.50; H, 5.46; N, 10.85; S, 12.41. Found: C, 46.58; H, 5.49; N, 10.84;S, 12.34.

(g)(2S*,5R*)-4-Ethoxy-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

To a solution of the product from step (f) (0.30 g, 1.16 mmol) inpyridine (0.19 mL, 0.18 g, 2.32 mmol) was added butyric anhydride (0.37mL, 0.36 g, 2.32 mmol) and the resulting mixture was stirred at ambienttemperature for 2 hours. Saturated NaHCO₃ /H₂ O was added and, after 1hour, the mixture was extracted with EtOAc (×2) and the combinedextracts were dried (Na₂ SO₄), concentrated in vacuo and flashchromatographed (EtOAc/hexanes, 1:1) to give the desired product (0.21g, 55%) as a yellowish solid: mp 59-61° C.; ¹ H NMR (CDCl₃): δ0.96 (t,J=7.4 Hz, 3H), 1.36 (t, J=7.1 Hz, 3H), 1.68 (sextuplet, J=7.4 Hz, 2H),1.80 (br s, 1H), 2.36 (t, J=7.4 Hz, 2H), 3.14 (dd, J=12.3, 3.5 Hz, 1H),3.59 (dd, J=12.3, 5.2 Hz, 1H), 4.40 (m, 3H), 4.59 (dd, J=12.3, 5.2 Hz,1H), 5.36 (dd, J=5.2, 3.4 Hz, 1H), 5.89 (d, J=7.3 Hz, 1H), 6.34 (dd,J=5.2, 3.9 Hz, 1H), 7.91 (d, J=7.3 Hz, 1H); MS m/z 329 (M=1, 11%), 141(100%). Anal. Calcd. for C₁₄ H₂₀ N₂ O₅ S: C, 51.21; H, 6.14; N, 8.53; S,9.76. Found: C, 51.08; H, 6.15; N, 8.39; S, 9.69.

(h)(2R,5S)-4-Ethoxy-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one

To a solution of the product from step (g) (10 mg) in 20% CH₃ CN/buffer(3.0 mL, 0.05 M. pH 8.0, phosphate) is added PLE (pig liver esterase,1.5 μL, Sigma) and the mixture is stirred at ambient temperature for 24hours. The aqueous solution is extracted with hexane (×2) and thecombined extracts were dried (Na₂ SO₄) and concentrated in vacuo to givethe desired product. HPLC analysis of the organic phase indicates thepresence of a single enantiomeric butyrate ester: The enantiomericalcohol is detected in the aqueous phase.

(i) (2R,5S)-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine

A solution of the ester from step (h) (4 mg) in NH₃ /MeOH (2 mL) isplaced in a steel bomb with a teflon liner, sealed and heated at 70° C.for 18 hours. Solvent is evaporated in vacuo to give the desired productwith HPLC, ¹ H NMR and MS properties identical to those of an authenticsample.

EXAMPLE 3(2S*,5R*)-5-Fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine

A solution of the product from step (g) (10 mg) in NH₃ /MeOH (2 mL ofMeOH saturated with NH₃ gas at 0° C. for 45 minutes) was placed in asteel bomb with a teflon liner, sealed and heated at 70° C. for 18hours. Solvent was evaporated in vacuo and acetone added to give thedesired product (8.8 mg, 99%) as a white solid: mp 195-196° C.; ¹ H NMR(DMSO-d₆): δ3.10 (dd, J=12.0, 4.2 Hz, 1H), 3.40 (dd, J=12.0, 5.3 Hz,1H), 3.70 (ddd, J=12.0, 5.5, 3.5 Hz, 1H), 3.77 (ddd, J=12.0, 5.5, 3.5Hz, 1H), 5.16 (t, J=3.5 Hz, 1H), 5.39 (t, J=5.5 Hz, 1H), 6.11 (m, 1H),7.56 (br s, 1H), 7.80 (br s, 1H), 8.17 (d, J=7.4 Hz, 1H); MS m/z 248(M+1, 34%), 130 (100%). Anal. Calcd. for C₈ H₁₀ FN₃ O₃ S: C, 38.86; H,4.08; N, 17.00; S, 12.97. Found: C, 38.97; H, 4.05; N, 16.96; S, 12.95.

EXAMPLE 4 (2S*,5R*)-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]cytosine

A solution of(2S*,5R*)-4-ethoxy-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one(0.21g) in ammonia/methanol (8 mL of methanol saturated with ammonia gasat 0° C. for 45 minutes) was placed in a steel bomb with a teflon liner,sealed and heated at 70° C. for 18 hours. Solvent was evaporated invacuo and the residue subjected to flash chromatography to give thedesired product (0.16g, 89%) as a white solid: mp 184-185° C.; ¹ H NMR(DMSO-d₆): δ3.00 (dd, J=11.8, 5.0 Hz, 1H), 3.38 (dd, J=11.8, 5.5 Hz,1H), 3.63-3.80 (m, 2H), 5.15 (t, J=4.5 Hz, 1H), 5.30 (t, J=6.0 Hz, 1H),5.70 (d, J=7.3 Hz, 1H); 6.18 (t, J=5.0 Hz, 1H), 7.20 (brd, 2H, NH₂),7.79 (d, J=7.3 Hz, 1H); MS m/z 229.8 (M+1, 4%), 112(100%). Anal. Calcd.for C₈ H₁₁ N₃ O₃ S: C, 41.91; H, 4.84; N, 18.33; S, 13.99. Found: C,41.97; H, 4.83; N, 18.24; S, 13.93

What is claimed is:
 1. A process for the preparation of compounds offormula (I) ##STR11## wherein R is hydrogen, C₁₋₆ alkyl, or halogen andY is hydroxy, amino, C₁₋₆ alkoxy or OR¹, where R¹ is a chiral auxiliary;which process comprises treating a compound of formula (II) ##STR12##wherein R and Y are as hereinbefore defined and R² represents hydrogen,C₁₋₆ -acyl, C₁₋₆ -alkyl or halogen with a suitable Lewis acid or areagent apt to convert the group OR² to a leaving group followed bytreating with an aqueous base.
 2. A process according to claim 1 whereinthe Lewis acid is stannic chloride or trimethylsilyl triflate.
 3. Aprocess according to claim 2 wherein the Lewis acid is stannic chlorideand the treatment is carried out at reduced temperature in a polaraprotic solvent.
 4. A process according to claim 1 wherein the compoundof formula (II) is prepared by reacting a compound of formula (III)##STR13## wherein R is hydrogen, C₁₋₆ alkyl, or halogen, and Y ishydroxy, amino, C₁₋₆ alkoxy, or OR¹ where R¹ is a chiral auxiliary, with1,4-dithiane-2,5-diol and, if necessary or desired, derivatisation.
 5. Aprocess according to claim 4 wherein the reaction with1,4-dithiane-2,5-diol is carried out at elevated temperature in anon-polar aprotic solvent.
 6. A process according to claim 5 wherein thereaction with 1,4-dithiane-2,5-diol is carried out at about 100° C. inanhydrous toluene.
 7. A method for obtaining a compound of formula (IIa)wherein R is H, R² is H, C₁₋₆ acyl, C₁₋₆ alkyl or halogen and Y is OR¹wherein R¹ is a chiral auxiliary, from a mixture of isomers by treatmentof the mixture of isomers, at least partially in solution, with an agentcapable of effecting interconversion of the isomers without completesuppression of the crystallization of the desired single enantiomer(IIa) wherein R is H and Y is OR¹.
 8. A compound of formula (II)##STR14## as defined in claim 1, which compound is selected from thegroup consisting of2-{[(4-ethoxy-5-fluoro-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ol,2-{[(4-ethoxy-2-pyrimidinyl)-oxy]methyl}-1,3-oxathiolan-5-ol,2-{[(4-ethoxy-5-fluoro-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ylacetate and 2-{[(4-ethoxy-2-pyrimidinyl)oxy]methyl}-1,3-oxathiolan-5-ylacetate.
 9. A compound of formula (IIa) ##STR15## wherein R representshydrogen, C₁₋₆ alkyl or halogen, R² represents hydrogen, C₁₋₆ acyl, C₁₋₆alkyl or halogen and Y represents OR¹ wherein R¹ represents (d)-menthyl,(I)-menthyl, (d)-8-phenylmenthyl, (I)-8-phenylmenthyl, (+)-norephedrineor (-)-norephedrine.
 10. A compound as claimed in claim 9 wherein R¹represents (I)-menthyl.
 11. A compound of formula (I) ##STR16## asdefined in claim 1, which compound is selected from(2S*,5R*)-4-ethoxy-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-oneand(2S*,5R*)-4-ethoxy-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one.12. A compound selected from the group consisting of(2S*,5R*)-4-ethoxy-5-fluoro-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-oneand(2S*,5R*)-4-ethoxy-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one.13. A compound selected from the group consisting of(2S,5R)-4-ethoxy-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-oneand(2S,5R)-4-ethoxy-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one.14. A compound selected from the group consisting of(2R,5S)-4-ethoxy-5-fluoro-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-oneand(2R,5S)-4-ethoxy-1-[2-(butanoyloxymethyl)-1,3-oxathiolan-5-yl]pyrimidin-2-one.15. A process for the preparation of a compound of formula (Ia)##STR17## wherein R is hydrogen, C₁₋₆ alkyl, or halogen, and Y ishydroxy, amino, C₁₋₆ alkoxy or OR¹, where R¹ is a chiral auxiliary,which process comprises treating a compound of formula (IIa) ##STR18##wherein R² is hydrogen, C₁₋₆ acyl, C₁₋₆ alkyl or halogen with a suitableLewis acid or a reagent apt to convert the group OR² to a leaving group,hydrolyzing the product with an aqueous base and, optionally, where R¹is not a chiral auxiliary esterifying the racemic β-diasteromer,enzymatically hydrolyzing the racemic ester and recovering the desiredenantiomer (IIa).